In a typical radio communications network, wireless communication devices, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more Core Networks (CN). The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a radio base station (RBS), which in some networks may also be called, for example, a “NodeB” or “eNodeB”. A cell is a geographical area where radio coverage is provided by the radio base station at a base station site or an antenna site in case the antenna and the radio base station are not collocated. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. One base station may have one or more cells. A cell may be downlink and/or uplink cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
A Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity. In some versions of the RAN as e.g. in UMTS, several base stations may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS) have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access technology wherein the radio base station nodes are directly connected to the EPC core network rather than to RNCs. In LTE the base station nodes are for example directly connected to Mobility Management Entities (MME). The MME is responsible for idle mode UE tracking and paging procedure including retransmissions. In general, in E-UTRAN/LTE the functions of a RNC are distributed between the radio base stations nodes, e.g. eNodeBs in LTE, and the core network. As such, the Radio Access Network (RAN) of an EPS has an essentially “flat” architecture comprising radio base station nodes without reporting to RNCs.
Machine Type Communications (MTC) is an area within telecommunications, sometimes also referred to as M2M or Internet of Things (IoT), in which it is envisioned that all types of devices which may potentially benefit from communicating will do so. That is, everything from agriculture and/or industrial sensors and actuators to things in the smart home or workout gauges in the personal networks will be connected wirelessly. Many devices may not, like a smart phone, be charged frequently and therefore it is beneficial if many of the MTC wireless communication devices, or UEs, try to conserve energy and power by sleeping as much as possible, i.e., having as much of their circuitry turned off as much as possible. In wireless communications networks, such as in 3GPP LTE networks, the UEs may save power by a discontinuous reception (DRX) mechanism. During DRX the UE may keep its receiver circuitry powered off to save power, but during a DRX sleep the network may not reach the UE to inform it about incoming data traffic, system information updates, etc.
When the network tries to send data traffic to the UE, the network uses a paging procedure, where a paging message is sent from the Core Network (CN) through the base station to the UE. The UE is expected to listen to the paging channel at certain time instants which are calculated using UE identification number (IMSI) and the current System Frame Number (SFN) in the cell. For a UE in idle mode this procedure is specified in 3GPP TS 36.304. In LTE, the SFN denotes a radio frame, which is 10 ms in length, comprising 10 subframes. The available SFN number goes from 0 to 1023. The full SFN cycle length is 10.24 seconds, therefore the SFN wraps around every 10.24 seconds in each cell. The UE should be awake at least during one paging occasion during a DRX cycle so the network paging message may reach the UE. The paging occasion is defined as the subframe during which the UE shall monitor the paging message. The paging occasion and Paging Frame (PF) is determined based on the length of the DRX cycle of the UE and based on an identification of the UE, such as a UE ID in LTE.
When a UE wakes up in a cell and does not know the downlink timing of the radio frames, the subframes and the symbols, it needs to obtain synchronization to the network first. There are different technology dependent ways to achieve these synchronizations. In LTE, in order to calculate the paging frame and the paging occasion the SFN synchronization is obtained. This may be achieved by reading the broadcasted Master Information Block (MIB) which includes information on the current SFN. If the UE sleeps for a very long period, its internal clock may experience clock drift large enough to require the UE to read the MIB again after the UE has woken up.
In the current LTE system the paging cycle length is at most 2.56 seconds. There is incentive to extend this cycle length in order to save more power especially for MTC UEs.
If the DRX cycle length or the paging cycle length of the UEs is extended beyond the SFN range, the current procedures of determining the paging occasion are not applicable any more.
Moreover, the SFN is not synchronized across different cells. Thus, if the UE moves to another cell while it is sleeping, this will also lead to problems related to the calculation of the correct wake-up time for paging, especially if the maximum paging cycle is extended.